The present invention relates to an apparatus which may be used in a microscope for producing an optical image contrast. In particular the apparatus includes a first screen, a projection arrangement for producing an image of the first screen in an object plane, a first device for producing a first relative movement between the image of the first screen and the object, a second screen which is geometrically similar to the first screen and a system for imaging the object plane into the plane of said second screen, wherein the imaging scale is chosen so that the image of the first screen in the plane of the second screen is congruent with the second screen.
Classical optical image production has limits with regard to depth of focus and resolution. These limitations have been improved upon by modern arrangements which utilize, among other things, the Moire effect and scanning methods, which when combined with optical converters and electronics avoid the requirements of direct image observation with the human eye.
DE-Al 35 27 074 discloses a device, preferably intended for use in stereo surgical microscopes, with a projection grating, and a reference grating for producing a Moire pattern. The relative position of the projection grating and reference grating is preferably changed by periodic tilting of a plane-parallel glass plate located adjacent to the grating which serves as both projection and reference grating. The apparatus is used for determining the nature of the surface of objects by means of contour lines, which are produced by the Moire effect.
The known arrangement is not suitable for instruments with high microscope resolution and high depth of focus requirements. As already mentioned, the projection grating and reference grating are united in a fixed component and the relative movement is produced by periodic tilting of a downstream optical element. Since the reference grating is not moved in relation to the object, the entire object area cannot be observed at one time. Only the part imaged onto the reference grating apertures can be observed at a given time.
DE-Al 23 60 197 and G. Q. Xiao, G. S. Kino, SPIE Vol. 809, Scanning Imaging Technology 1987, page 107, disclose confocal optical scanning microscopes with simultaneous observation of a plurality of object points. In these cases, a rotating dot screen disk (Nipkow disk) is used. For illumination, the disk is imaged onto the object. For detection, the object is imaged back onto the disk. Again, analogous to the use of a single grating in DE-Al 35 27 074, the projection screen and reference screen are united in a single component.
The confocal arrangement has the effect of improving resolution in the object plane and increasing the depth of focus compared with classical microscopes. The simultaneous observation of many points has the effect of achieving better light efficiency and a faster image composition compared with known single-beam scanning microscopes.
Because the illumination beam and detection beam are combined between the disk and object, separating the detection light from the unwanted light is a great problem and, as a result, there are effective light losses in the detection. An arrangement by Petran et al, cited in Xiao et al, based on the same principle, uses in each case two holes from different areas of the rotating disk for the illumination beam and the detection beam. As a result, the production of a sufficiently exact Nipkow disk and the adjustment of the optics are made considerably more difficult, since the beam from the hole on the illumination side via an object point must always strike precisely at a hole on the detection side, to be precise for all holes of the disk.
Compared with the system by Petran et al., one advantage of the present invention is that it is possible to dispense with the extreme demands on the symmetry of the grating disks -- on each grating aperture a second must fit exactly at a certain angular distance -- and on the adjustment of the optics. In addition, the requirement according to the present invention, that the image of the first screen in the plane of the second screen be congruent with the second screen, is, on the other hand, much easier to realize and does not have to be maintained very closely.
Compared with the systems according to Xiao et al. and according to DE-Al 23 60 197, one advantage of the present invention is that the light filtered out by the grating on the illumination side quite obviously cannot reach the detection system. Therefore, problems of constant light suppression are considerably reduced by the present invention. In addition, only in the present invention is constant light suppression effectively achieved by the simple method of forming the difference between two images.